Conventional switched capacitor input systems, such as for integrator circuits in sigma delta modulators, employ a reference voltage, reference capacitor and input capacitor to determine an unknown input voltage. A switching system applies the reference voltage to charge the reference capacitor and the input voltage to charge the input capacitance in a first phase, then transfers those charges to a summing node of the integrator circuit in a second phase. At the end of the second phase the integrator output reflects the sum of the input and reference charge. The output of the integrator circuit resulting from that action is delivered to a comparator. At the end of the second phase the capacitor is clocked and provides at its output one level or another 1 or 0, which is fed back to control whether the reference voltage applied to the reference capacitor should be positive or negative. The average of the 1's and 0's over time is thus a representation of the unknown input voltage.
A variation of that approach employs a differential integrator with two input capacitors that receive positive and negative unknown input voltages and two reference capacitors to obtain a mutual offsetting of any errors in the on-chip circuitry. A further improvement modifies the switching circuit in order to operate the system on a single polarity reference voltage U.S. Pat. No. 5,323,158, Switched Capacitor One-Bit Digital-to-Analog Converter, Paul F. Ferguson. One example of this is to add another level of switching to alternately effect the charge transfer between the reference capacitors and selective ones of the positive and negative summing nodes of the differential integrator circuit.
In a further improvement that increases efficiency by operating fully in both phases, the input capacitors are connected to alternative summing nodes in each phase. This achieves charge transfer from the input capacitors to the integrator capacitors in both phases instead of in the one phase only.
An alternative application of switched capacitor input sigma delta modulators is as capacitive sensing devices where the input voltage is now known and the capacitance is variable and unknown. In that application the 1, 0 comparator output of the modulator represents the value of the unknown capacitance.
One shortcoming of these devices is that unknown variable capacitance in practical applications frequently suffers from leakage current which introduces error into the capacitance determination. This is particularly a problem when the unknown capacitance is remote e.g. off-chip.